Use API method and trigger event when user is created in unittests and behat

When using data generators in behat and unittests, users are not created properly but instead we just insert a record in the database.
In MDL-59791 Tim also noticed that custom profile fields are not created.

This issue suggests to use API methods when creating users.

There can be various things happening in plugins on user creation, in workplace, for example, we allocate to tenants, execute dynamic rules, etc. All this is not possible with the current generator.

Also I noticed quite a few errors in existing unittests when users are created with invalid data and also deleted users for some reason don't have context. This makes some existing unittests not correct too.